Agarwood oil contains many unusual substances. The
elucidation of the structures of many minor components of agarwood oil,
especially of Far Eastern origin, have kept many structural organic
chemists busy for years, and continue to do so [see Lawrence (1998) for a
review of progress from 1973-1998].

Indian
Agarwood

The presence of eight
sesquiterpenes including agarol and agarospirol together with a-
and b-
agofuran, were characterised by Battacharrya et al. in 1959 & 1965
[Jain & Battacharrya (1959), Varma, Maheshwari & Battacharrya (1965)].
Maheshwari et al. (1963, 1963a) characterised alpha- and beta-agaofuran,
dihydroagarofuran, nor-ketoaragofuran, 4-hydroxydihydroagarofuran and 3,4
dihydroxydihydroagarofuran by solvent extraction of infected agarwood of
Indian origin. Subsequent total synthesis of nor-ketoagarofuran by Heathcock
& Kelly (1968) showed the formula presented by Maheshwari et al. was
incorrect. Pant & Ragosti (1980) used instrumental techniques to show that
dihydroagarofuran was dihydro-beta-agarofuran and isodihydroagarofuran was
isodihydro-alpha-agarofuran.

Pant & Rastogi (1980) also
confirmed the presence of two sesquiterpenes: agarol & gmelofuran in the
fragrant wood. Ishihara et al. (1991a) described seven newly
identified compounds in agarwood oil from A. agallocha based on the
guaiane skeleton, including guaia-1(10),11-dien-15-ol,
guaia-1(10),11-dien-15-carboxylic acid,
methylguaia-1(10)-11-dien-15-carboxylate, guaia-1(10),11-dien-9-one,
1,1`0-epoxyguaia-11-ene, guaia-1(10),11-dien-15,2-olide and
guaia-1(5),11-dien-2-one. In a subsequent paper Ishihara (1991b) described a
further 3 sesquiterpenes and ascribed odour descriptions to some
characterised components.

Näf et al. (1992,
1995) described components in fresh agarwood oil distilled from some freshly
purchased Indian wood (identified as A. agallocha), The authors
identified beta-agarofuran, vetispira-2(11),6(14)-dien-7-ol, dihydrokaranone
and valerianol as major constituents of Indian agarwood oil. Eight new
valencane-, eremophilane- and vetispirane- derivatives were described (see
abstract in bibliography) including (3R, 7R, 9R,
10S)-9,10-dimethyl-6-methylene-4-oxatricyclo[7.4.0.03,7]1-tridecene,
and compounds synonymous with 2,14-epoxyvetaspira-6-ene and
2,14-epoxyvetaspira-6(14),7-diene were also described (1995), odour
descriptions being ascribed to three of the characterised compounds:

Amongst other components, an
Indian patent reveals the presence of 1,7-oxoaporphine and liriodenine in
the heartwood [Indian Patent (1979) 145857 through Chem Abstr. (1980)
93, 80049f]. Bhandari et al. (1982) confirmed the presence of a
coumarinolignan, aquillochin, from A. agallocha.

Indonesian
Agarwood (A. malaccensis/A. agallocha).

Jinkohol (2b-hydroxy-(+)-prezizane)
was characterised by Nakanishi et al. (1980) following work on a
benzene extract of Indonesian agarwood imported via Singapore.

In 1983, two new
sesquiterpenes: jinkoh-eremol and jinkohol II were characterised by the same
author from “Aquliaria sp; (not A. agallocha) probably A.
malaccensis Benth.” from agarwood “collected in Indoneisa and imported
via Singapore ”, which they called “type” B agarwood, reserving the
description “type A” for agarwood from A. agallocha.

The authors reasoned that
“type B” agarwood contained nootkatane (e.g. kusunol, jinkoh-eremol),
spirovetivane (e.g. agarospirol) and tricyclic (+)-prezizane (e.g. jinkohol,
jinkohol II) type sequiterpenes, and that these structural types also occur
in vetiver oil, and may evolve from a common precursor. The authors
also indicated that four compounds possessed intense odours (agarospirol,
kusunol, jinkohol & jinkohol II) and “appear to be the source of the
fragrance of agarwood”.

Yoneda et al. (1984)
were able to list the major sesquiterpenes of agarwood from type A and type
B agarwoods imported from Indonesia and Vietnam through Singapore. A.agallocha (agawood type A) was found to contain b-agarofuran
0.6%, nor-ketoagarofuran 0.6%, agarspirol 4.7%, jinko-eremol 4.0%, kusunol
2.9%, dihydrokaranone 2.4%, and oxo-agarospirol 5.8%. In type B agarwood the
following compounds were identified a-agarofuran,
(-)-10-epi-g-eudesmol
6.2%, agarospirol 7.2%, jinkohol 5.2%, jinko-eremol 3.7%, kusunol 3.4%,
jinkohol II 5.6%, and oxo-agarospirol 3.1%. From their findings the authors
concluded that type A wood exclusively contains nor-ketoagarofuran and
dihydrokaranone, but does not contain (-)-10-epi-gamma-eudesmol, jinkohol
and jinkohol II, findings which might be used to distinguish the products.

Yang et al. (1989)
from an ether fraction of an alcohol soluble extract of A. sinensis
distinguished a new chromone, 6-hydroxy-2-[2-(4'-methoxylphenyl)ethyl]
chromone and the other five known chromones: 2-(2-phenylethyl)chromone,
6-methoxy-2-(2-phenylethyl)chromone ,
6.7-dimethoxy-2-(2-phenylethyl)chromone ,
6-methoxy-2[2-(3'-methoxyphenyl)ethyl]chromone and
6-hydroxy-2-(2-phenylethyl) chromone. Later Yang & Wang (1990) elucidated
the structure of three 2-(2-phenylethyl) chromone derivatives isolated from
the ethyl acetate soluble fraction of the alcoholic extract of Aquilaria
sinensis (Lous.) Gilg. These were:
5,8-dihydroxy-2-(2-p-methoxyphenylethyl) chromone and
6,7-dimethoxy-2-(2-p-methoxyphenylethyl) chromone. This was followed by
Yagura (2003) who identified 5-hydroxy-6-methoxy-2-(2-phenylethyl)chromone,
6-hydroxy-2-(2-hydroxy-2-phenylethyl)chromone,
8-chloro-2-(2-phenylethyl)-5,6,7-trihydroxy-5,6,7,8-tetrahydrochromone,
6,7-dihydroxy-2-(2-phenylethyl)-5,6,7,8-tetrahydrochromone were isolated
from a methanol extract of withered wood of Aquilaria sinensis.

Hsu (1996) notes the presence
of agarospirol, agarol, agarofuran,dihydroagarofuran,
4-hydroxyagarofuran, 3,4-hydroxydihydroagarofuran, and norketoagarofuran in
infected extracted wood, and the presence of benzylacetone, p-methoxybenzylacetone,
hydrocinnamic acid, p-methoxyhydrocinnamic acid, agarospirol, agarol,
agarofurans and agarotetrol in the essential oil, the inference being this
is from non-infected wood.

Thai Agarwood.

The presence of the cytotoxic
compounds 1,3-dibehenyl-2-ferulyl glyceride, which is novel, and
12-O-n-deca- 2, 4, 6-trienoylphorbol-13-acetate isolated from the stem bark
of an A. malaccensis tree growing in Thailand was demonstrated by
Gunasekera et al
(1981)

Vietnamese Agarwood: Aquilaria
agallocha.

Ishara et al. (1993)
found the following compounds, amongst others, in a Vietnamese agarwood
extract, describing their odours as follows:

and in a further paper Ishara
et al. (1993b) describe the volatiles from extracts of 4 different
agarwoods, concluding that major sesquiterpene volatile in Kanankoh (Murrasaki
from Vietnam) was oxo-agarospirol, and from two samples of Kanankoh (Ryoku-yu
& Cha-yu – both from Vietnam) the major volatiles were the compounds below:

In a third
study Ishara et al. (1993c) looked at the volatiles from ground
agarwood from a heated flask, capturing the volatiles in a Tenax trap. Clear
differences in trapped volatiles composition were seen between Kanankoh (Ryoku-ku)
and Jinkoh (Bateikel) agarwoods, with the latter showing a higher propotion
of lower molecular weight materials such as 3,4-dimethoxyphenol,
benzylacetone and benzaldehyde, but with a more complex picture with higher
molecular weight materials. Lower levels of jinkoh-eremol, kusunol and
guaia-1(10),11-dienal were shown by the Jinkoh (Bateikel) agarwood, but it
had higher levels of beta-agarofuran, neopetasane and dihydrokaranone.